Radio receiving circuit



H. P. KALMUS RADIO RECEIVING CIRCUIT Filed Nov. 1, 1939 March 4, 1941.

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. Henry P-K'almua Afi'arrwgfi.

Dnshmc Patented Mar. 4, 1941 This invention. relates to improvements inradio receiving circuits and particularly those employed insuper-heterodyne radio receiving An object of this invention is theprovision of an improved super-heterodyne receiving circuit whereby itis adapted to receive frequency modulated signal waves.

An important object of this invention is the provision of an improvedform of circuit of the above type which has been simplified whileimproving the eillciency thereof.

A still further object of the invention involves the simplification ofthe so-called discriminating and limiting portions of a super-heterodyneradio receiving circuit adapted to respond to frequency modulatedcarrier waves.

A more specific object ofthe invention is to provide in an arrangementof .the above type a single vacuum tube device and associated circuitsfor performing both the limiting and discriminating functions infrequenc ing circuits.

These and other objects as will appear from the following disclosure aresuccessfully secured by means of the invention herein described.

This invention resides substantially in the combination, construction,arrangement and relative location of parts, all as will be detail below.

In the accompanying drawing:

Figure 1 is a diagrammatic illustration of that portion of asuper-heterodyne radio receiving circuit adapted to receive frequencymodulated waves illustrating the subject matter of this invention; andFigure 2 is a chart of the potential distribution within the vacuum tubeof this invention which is employed in accordance therewith to 40accomplish both the limiting and discriminat-' ing functions of suchapparatus.

As is known in the art any radio receiving circuit adapted to reproducethe modulating current, which is of course the current representative ofthe signal, of a frequency modulated carrier, commonly employs portionsof the circuit for limiting the amplitude of the signal wave and fordemodulating the limited wave to produce an audio -frequency signalcurrent which is of course the original modulating current. In the priorart arrangements for accomplishing this purpose the output of theintermediate frequency amplifier of a super-heterodyne receiver is fedinto a pentode vacuum tube, the output current of which has beenlimitedin amplitude. This y modulated receivdescribed in full UNITED STATESPATENT OFFICE A 2,233,706

l I RADIO RECEIVING CIBQUI'I Henry P. Kalmus, New York, N. Y., minor to"Emerson Radio & Phonograph Corporation,

New York, N. Y., a corporationof New York Application November 1, 1939,Serial No. 302,285

portion of the circuit distinct from the limiter o portion referred toabove.

In accordance with this invention substantial simplification of thediscriminating portion of the circuit is accom'plshed. A simple resonantcircult and a suitable multi-grid vacuum tube takes 15 the place of thepentode previously used and the limiting and the discriminatingfunctions are performed thereby with further attendant ad-' vantageshereinafter to be described.

At I is diagrammatically illustrated a portion 20 of the last vacuumtube of the intermediate frequency amplifier commonly employed in asuperheterodyne receiving set. The output of this tube is fed through asuitable radio frequency transformer 2 to the input circuit of themulti- 25 grid tube generally indicated by the reference numeral 1. Itmay be assumed for purposes of description that the intermediatefrequency is 3 megacycles. It follows therefore that the input circuitwill be tuned to a frequency of 3 meg- 3 acycles. The input circuit ofthe multi-grid tube includes the secondary of the transformer 2 shuntedby a suitable capacitor 3 with the values thereof fixed to render thecombination resonant at 3 megacycles. One terminal of this com- 35bination of inductor and capacitor is connected by Wire 4 through acapacitor 5 to the control grid 6 of the multi-grid tube 1. The otherter-- minal of the resonant circuit is connected by the wire: 8 to thecathode return lead 9. The 40 cathode l3 of the tube is connected by awire In through a suitable resistor H to the cathode return 9. Theresistor H is shunted by a capacitor I2 of suitable value to by-pass theradio frequencies present. A suitable grid leak resistor 34 is connectedbetween the cathode I3 and the control grid 6.

As diagrammatically illustrated the multigrid tube 1 may include a gridll, adapted to be connected by a wire l5 to the positive side of asuitable potential source so as to maintain this grid positive withrespect to the cathode in accordance with usual practice. A suitablehigh frequency by-pass circuit is'provided' between the lead l5 and thecathode return 9 by means 66 of a properly proportioned capacitor I8.The multi-grid tube 1 has a third grid I1, a fourth grid I8, and a fifthgrid I9. The grids I| and I9 are provided with a common lead 26 which 5is connected to the positive terminal of a suitable potential source anda by-pass capacitor 2I is provided between the lead 28 and the cathodereturn 9. The grid I8 is connected to the return lead 9 by a lead 22througha resonant'circuit comprising the inductor 28 and the capacitor24. This circuit is tuned to resonance'at 3 megacycles'- It may benoted, as indicated diagrammatically, that the grids 6, I4, I'l, I8 and,I9 are spatially distributed within the envelope of the multi-grid'tubein the manner illustrated. The distributionof the elements and the pitchof the The plate or anode 25 of the tube 'l'connected by a wire 26.through a suitable resistor 21 of the order of 50,000 ohms to thepositive side of the plate current source and lead 26 is connectedthrougha suitable by-pass capacitor 28 to the cathode return lead 9.Wire 26 'is connected by a wire through a capacitor 29, a high resistor30, and a resistor 3| of lower value to the return lead 9. The commonpoint of resistors 38 and 3I is connected through a capacitor 32 to thereturn lead 9. A suitable adjustable connection 33 togetherwith thereturn lead 9 provide the output terminals for this apparatus whichwould be coupled in any suitable manner to an audio signal responsivedevice such as a loud speaker or an audio frequency amplifier ifamplification is desired.

Figure 2 illustrates diagrammatically the potential distribution in themulti-grid tube I betweenthe cathode and plate along a theoretical linewhich does not cut any of the turns of the grids. When grid I8 isnegative the electrons 40 passing through grids 6 and "are partiallyimpeded with the result that a virtual cathode is formed before the gridI8. During the time grid 8 is positively charged this virtual cathodecauses a bending in the curve of potential between grids I1 and I8 asindicated in Figure 2. During the,

sentsthe potential distribution from cathode to plate'when grid 6 ispositive and for difierent positive values thereof with the variationindicated in the dotted line in the region between grids I1 and I8during the negatively charged time of grid 6. This chartserves tofurther demonstrate that the space charge periodi-callyformed beforegrid I8 periodically charges that grid.

' .65 If it is. imagined that grid I8 is connected to the cathodethrough a condenser this induced charge causes a periodic charging ofthe condenser and a simultaneous alternating current voltage betweengrid I8 and the cathode. changing at the. same rate as the space chargeand the alternating current voltage on grid 6.

When this is at maximum positive value most of the electrons are crowdedbefore grid I8, and this grid becomes negative if there is a capacity 75reactance between grid I8 and cathode. There- This voltage is I fore,the alternating current voltage introducedbetween grid "and the cathodeis Just out of phase with the alternating current voltagebetween grid 6and the cathode. As stated another way, it can be imagined that there isan 5 apparent negative capacity between grids 8 and I8. There is no suchapparent negative capacity in the opposite direction between grids I8and 6. Therefore, the described effect is comparable with a' negativecapacity in one direction. By reason of these characteristics it ispossible by connectinga resonantcircuit proper- 1y tuned between thegrid I8 and ground to perform the discriminating functions desired in apparatus of this type when adapted to the reception of frequencymodulated signal waves.

In order to understand the operation of this circuit the action whichgoes on in the multigrid tube needs some description. Before giving thisdescription it will be noted, however, as 20 is apparent to thoseskilled in the art, that all of the circuit connections for the portionof the circuit shown have not been completed because their completion isapparent to those skilled in the art, and by omitting certain portionsthe disclosure is thereby simplified. As ndted above, the grid 8 is thecontrol or input grid of the tube, which gn'dis initially grid leakbiased so as to be negative with respect to the cathode. The grids I4,I1 and I9 are maintained positive with respect to the cathodes,whereas-the grid 3 I8 is made negative with respect to the cathode byresistor II. A space charge is present between grids I1 and I8 dependingon grid 6 potential. This space charge is varying in accord-' 35 ancewith the alternating current voltageon grid 6. This variation of thespace charge produces an alternating current voltage on grid I8 bycapacitive induction. If there is a capacity reactance between grid I8and cathode return lead 9 this capacity is charged and discharged in thesame phase as the space charge varies at grid I8. In other words, thealternat ing current voltage produced on grid I8 of the same frequencyas that present on grid 8 is.180 degrees out of phase with grid 6voltage, thereby decreasing the slope of, the characteristicof the platecurrent, as well as the plate current of the tube. If an inductivereactance is present" a between grid I8 and cathode return lead"9 thevoltages on grids 6 and I8 will be in phase, thereby, increasing theslope of'the characteristic of the plate current and therefore the platecurrent of the tube. On theother hand, if resistance is present betweengrid I8 and lead 9 the voltages of grids'Ii and I8 are ninety degreesout of phase so that. no change occurs in the slope of the plate currentcharacteristic. Therefore, if a resonant circuit including an inductorand acapacitor is inserted between the'grid I8 and return lead 9,,depending upon the relationship of the resonance frequency thereof andthe frequency ap-. plied to grid 6, the'useful results of this inventionmay be secured.

With such an arrangement the plate current changes progressively withthe frequency of the voltage variation of grid .6, i. e., the platecurrent changes proportionately'and in :the same. direction asthe inputfrequency. Hence, if, with. the frequency characteristics of theresonant circuit 23-24 fixed, the control grid frequency changes, theplate current changes in the samev way. The result is that the output ofthe circuit, as illustrated in Figure 1, is the same as the output-ofthe conventional discriminator previously mentioned. The slope of theplate current characteristic, however, when plotted against the inputfrequency is much greater by this arrangement, with the result that thisdevice is more sensitive and has greater output.

The arrangement also can act as a limiter, cutting oil, so to speak, thetops of the frequency modulated carrier, as well as all portions of ex-.traneous noises such as static above the amplitude of cut-off providingthe desirable limiting function oi a circuit of'this type. Limiting isaccomplished by grid circuit cut-oft due to condenser and grid leak 34and by plate circuit cut-oi! due to proper choice of voltage on andgeometry of grids I 4 and II, as is well known in the art.

From the above description it will be apparent to those skilled in theart that I have devised certain new and novel circuit combinations whichproduce advantages of practical importance and that the embodiment oithe invention herein disclosed for the purpose of illustrating theinvention is capable of variation without departure from the novel scopeof the subject matter herein' disclosed. I do not, therefore, desire tobe limited bythis disclosure but rather by the scope of the claimsgranted me.

What is claimed is:

1. In a combined radio frequency carrier operated limiting anddiscriminating circuit, the combination including a multi-electrodevacuum tube including a cathode, an anode and a plurality 01 controlelectrodes, a tuned input circuit including one of said controlelectrodes and said cathode, an output circuit including said anode andsaid cathode, means for biasing the control electrode independently ofthe other electrodes, means for biasing others of said controlelectrodes, the biasing of said biased electrodes being selected tolimit, the response to the amplitude of the input, and a resonantimpedance network connecting another of said control electrodes withsaid cathode whereby the current in said output circuit is varied by andin accordance with changes in the frequency on the first controlelectrode.

2. In a radio circuit combination, a vacuum tube having a cathode, ananode and a plurality of grids, an input circuit connected to one ofsaid grids and said cathode, an output circuit connected to said anodeand said cathode, means connected between said cathode and another ofsaid grids for causing the current in the output circuit to changeprogressively with changes in frequency in the input circuit, and meansfor biasing one or more of the grids to limit the response to theamplitude of the input.

3. In a combined radio frequency carrier operated limiting anddiscriminating circuit, the combination including a multi-electrodevacuum tube including a cathode, an anode and a plurality of controlelectrodes, a tuned input circuit including one of said controlelectrodes and said cathode, an output circuit including said anode andsaid cathode, means for biasing the control electrode independently oithe other electrodes, means for biasing others of said controlelectrodes, the biasing of said biased electrodes being selected tolimit the response to the amplitude.

of the input, and a resonant impedance network having a resistancecomponent connecting another of said control electrodes with saidcathode whereby the current in said output circuit is varied by and inaccordance with changes in the frequency on the first control electrode.

4. In a radio circuit combination, a vacuum tube having a cathode, ananode and a plurality of grids, an input circuit connected to one ofsaid grids and said cathode, an output circuit connected to said anodeand said cathode, means connected between said cathode and another orsaid grids for causing the anode current to change progressively withchanges in the frequency on the first grid, and means for self biasingthe input grid to limit the response to the amplitude oi the input.

5. In a radio circuit combination, a vacuum tube having a cathode, ananode and a plurality of grids, an input circuit connected to one oisaid grids and said cathode, an output circuit connected to said anodeand said cathode, a resonant impedance network connected between saidcathode and another of said grids for causing the current in the outputcircuit to change progressively with changes in the frequency in theinput circuit, and means for biasing the input grid, this biasing beingselected in relahon to the shape and spacing of said grids to responseto the amplitude of the input.

